The present invention relates to a method of determining an image recording device characterisation.
The use of image recording devices is well known to convert information upon an image into electronic form. Examples of these devices include scanners and digital cameras. Scanners are widespread in the photographic and image processing industries, in addition to being common in personal computing applications for domestic uses. Likewise, digital cameras are becoming increasingly popular for domestic use, in association with personal computers. It is important that such devices can accurately reproduce the information contained in an image, which in the case of a scanner is obtained from a scanned medium. This is a particular problem where the accurate reproduction of colours is required.
One problem is that the data representing uncorrected information from an image is generally not a colour accurate reproduction of the original scene or medium. There are various causes for these discrepancies and these are mainly produced by the image recording device apparatus itself such as by the light source, the device optics and the red, green and blue filters used in order to determine colour information. It is therefore common practice to generate a device “characterisation” which relates the colours within the image, which are device independent, to the colours as seen by the device (device dependent). Because of this, it is desirable to produce a characterisation for each device model, and preferably for each actual device produced, due to slight variations introduced into the apparatus during manufacture.
The device independent colours are normally represented in the form of X,Y,Z or L,A,B values whereas typically the colours scanned (device dependent) are represented as R,G,B values. These values are defined according to a standards body such as Commission Internationale de l'Eclairage (CIE).
In order to generate an accurate characterisation for a device, it is desirable to base this characterisation upon a wide range of device independent and dependent values. Therefore potentially all possible colours in the image which produce corresponding device independent data, can be accurately represented as appropriate device dependent data.
For scanners, such a characterisation can normally be achieved for many typical media using modern paint sets. However, in some instances a sufficient range of device independent values is not available and therefore it is not always possible to produce a suitable characterisation covering an appropriately broad range of X,Y,Z or L,A,B values. One particular example of this is in the reproduction of old paintings in which not only is the paint set no longer available, it may never have been accurately known and indeed the quality of the medium may have deteriorated over time.
In some instances it is therefore important to be able to produce accurate characterisations from a limited amount of data.
One method of achieving this is to perform a detailed analysis of spectra obtained from a medium such as an old painting. A paint set can then be physically generated and scanned in accordance with the standard practice for readily available paint sets. However, it will be appreciated that it is difficult and expensive to reproduce such paint sets, particularly as the corresponding dyes may no longer be available.